The present invention relates to an objective lens for recording or reproducing information on or from an optical information recording medium, in particular, an objective lens with less aberration, a method for correcting a manufacturing error thereof, and an optical pick-up device using the objective lens.
Light-utilizing technology has various characteristics such as high speed owing to high frequency, and capability of spacial information processing, phase processing, etc. Therefore, such technology has been under research and development, and used in practical applications in a variety of fields including communication, instrumentation, processing, and the like.
In the light-utilizing technology, a high-precision objective lens has been used as means for focusing an optical beam which is emitted from a light source and irradiated onto a recording medium. In recent years, the demand for an image recording device, etc. utilizing light has been especially increasing, and technology for realizing mass-storage recording is of increasing importance.
In order to record mass optical information, it is necessary to improve recording media so as to achieve high recording density, and what is more, to form a smaller beam spot, that is, to focus a beam spot sufficiently by an objective lens. In this manner, more information can be recorded by forming a smaller beam spot.
Besides, the diameter of a beam spot is proportional to the wavelength of light emitted from a light source to be used, and inversely proportional to the NA (Numerical Aperture) of an objective lens. Consequently, in order to form a smaller beam spot, it is necessary to shorten the wavelength of the light emitted from a light source to be used, or to increase the NA of the objective lens so as to obtain a higher NA.
As for the wavelength of light, a blue laser diode and a blue or green SHG (Second Harmonic Generation) laser are being developed, and shorter wavelength is being realized.
Meanwhile, as for the NAs of objective lenses, for example, the NA of a CD (Compact Disc) is 0.45, and the NA of a DVD (Digital Versatile Disc) is 0.6. In this manner, the DVD, having a higher NA than the CD, has achieved higher density of information to be recorded.
Besides, an optical pick-up device which uses a 2-group lens (a pair of lenses) so as to realize an objective lens having a further higher NA is disclosed in Japanese Unexamined Patent Publication No. 10-123410 (Tokukaihei 10-123410, published on May 15, 1998). In this optical pick-up device, an objective lens is made up of a 2-group lens in which a pair of lenses are combined using a lens barrel or the like. With this arrangement, the NA of the objective lens can be set at 0.85, achieving a higher NA.
However, when combining these two lenses in the above arrangement, an interval between the lenses, positioning of these lenses should be carried out accurately considering tilts of the respective lenses with respect to an optical axis, and a deviation of centers of the lenses, etc. Therefore, at the time of positioning, it is necessary to adjust the positions of the lenses while actually emitting a laser beam and checking the focusing condition of a beam spot and an aberration actually caused. Consequently, the positioning process is complicated.
In addition, resin is required to fix the 2-group lens. However, resin does not have sufficient heat and moisture resistance properties, and changes over time. In this manner, resin may have characteristic changes, and thus has low reliability.
Further, the 2-group lens weighs heavier as it includes the lens barrel, etc. Generally, focus control and tracking control are carried out by an actuator, by moving an objective lens in a direction parallel or vertical to the optical axis. Here, if the objective lens is heavy in weight, the objective lens cannot be driven at a high speed, making the information recording/reproducing speed slow.
On the other hand, an optical pick-up device using a single lens as an objective lens so as to achieve a higher NA is disclosed in Japanese Unexamined Patent Publication No. 9-311271 (Tokukaihei 9-311271, published on Dec. 2, 1997). As shown in FIG. 37, the optical pick-up device includes an objective lens in which a diffraction lens 103 and an aspherical lens 102 are integrated, a convex skew plate 104, and a concave skew plate 105.
In the foregoing optical pick-up device, a laser beam emitted from a light source is irradiated via the objective lens onto a disk 101, and thus information is recorded and reproduced on and from the disk 101.
With this structure, a chromatic aberration can be corrected by using the objective lens in which the diffraction lens 103 and the aspherical lens 102 are integrated. Besides, a comatic aberration can be corrected by moving the convex skew plate 104 and the concave skew plate 105 respectively in opposite directions.
In this manner, since a single lens is used as the objective lens of the optical pick-up device shown in FIG. 37, positioning process, resin for fixing lenses, etc. are not required.
However, the objective lens shown in FIG. 37 is made up by integrating the diffraction lens 103 and the aspherical lens 102. Therefore, when diffraction efficiency is reduced due to an error caused in a process for manufacturing the objective lens, lens transmittance of a laser beam (light) is decreased, resulting in a decline in light utilization efficiency.
Besides, since the interval of a diffraction grating of the diffraction lens 103 is very narrow, it is difficult to manufacture the diffraction lens 103. Using such a diffraction lens 103 involves an increase in manufacturing cost and characteristic changes depending on a polarization direction. Consequently, it is difficult to further downsize the objective lens provided with the diffraction lens 103.
It is therefore an object of the present invention to provide an objective lens which is downsized, has less aberration, and can correct the aberration easily, without a decline in light utilization efficiency; a method for correcting a manufacturing error thereof; and an optical pick-up device using the objective lens.
To achieve the foregoing object, an objective lens of the present invention is an objective lens having a numerical aperture not less than 0.75, and the objective lens is structured such that it is a single-lens objective lens both surfaces of which are aspherical, and is made of a material satisfying:
1.75 less than n, and 35 less than v, 
where n is refractive index for at least one of used wavelengths, and v is an Abbe number for a d-line.
According to the foregoing structure, a single-lens objective lens is used as the objective lens, and, for example, a diffraction element, etc. are not required. Therefore, efficiency to utilize light (laser beam) passing through the objective lens can be improved. In addition, manufacture of the objective lens can be facilitated, and further, miniaturization of the objective lens can be realized.
Besides, since the numerical aperture NA of the objective lens is not less than 0.75, for example, it becomes possible to form a smaller beam spot incident on a recording medium, realizing higher recording density in the recording medium.
Generally, a chromatic aberration is caused since the refractive index of a material constituting the objective lens varies depending on the wavelength of light. Due to the chromatic aberration, the position or the size of an image differs depending on the wavelength.
However, by setting as 35 less than v, the chromatic aberration can be lessened, and such an arrangement can prevent the phenomenon that an image point is shifted and a beam spot is out of focus due to a slight change in the wavelength.
Besides, by setting as 1.75 less than n, the curvature of a first surface of the objective lens can be reduced, facilitating the manufacture of the objective lens.
An optical pick-up device of the present invention is structured so as to include:
a light source; and
an objective lens having a numerical aperture not less than 0.75,
the objective lens being made of a material satisfying:
1.75 less than n, and 35 less than v, 
where n is a refractive index for at least one of used wavelengths, and v is an Abbe number for a d-line.
the objective lens being a single-lens objective lens both surfaces of which are aspherical,
wherein information is recorded or reproduced on or from a recording medium by irradiating light emitted from the light source onto the recording medium using the objective lens.
The foregoing structure can provide an optical pick-up device which has less chromatic aberration and spherical aberration, and can accurately condense a laser beam onto the recording medium, without a decline in the efficiency to utilize light (laser beam) passing through the objective lens.
A method for correcting a manufacturing error of an objective lens of the present invention includes the step of correcting a manufacturing error caused when manufacturing an objective lens having a numerical aperture not less than 0.75,
the objective lens being made of a material satisfying:
1.75 less than n, and 35 less than v, 
where n is a refractive index for at least one of used wavelengths, and v is an Abbe number for a d-line.
the objective lens being a single-lens objective lens both surfaces of which are aspherical,
by adjusting a tilt of the objective lens with respect to an optical pick-up device,
the optical pick-up device structured so as to include a light source and the objective lens, for recording or reproducing information on or from a recording medium by irradiating light emitted from the light source onto the recording medium using the objective lens.
Generally, when manufacturing an objective lens, considering the accuracy for positioning a mold at the time of molding, the accuracies of the weight and the volume of the preform, which is a material for the objective lens, the accuracy of processing the mold, etc., manufacturing errors such as the tilts of both surfaces of the objective lens, and a deviation of the centers of the both surfaces of the objective lens are caused. A comatic aberration is caused due to such manufacturing errors.
However, according to the foregoing structure, the comatic aberration caused by the manufacturing errors can be corrected by adjusting the tilt of the objective lens with respect to the optical pick-up device. Consequently, the aberration can be lessened, and the aberration can be easily corrected.
A method for correcting a manufacturing error of an objective lens of the present invention includes the step of correcting a manufacturing error caused when manufacturing an objective lens having a numerical aperture not less than 0.75,
the objective lens being made of a material satisfying:
1.75 less than n, and 35 less than v, 
where n is a refractive index for at least one of used wavelengths, and v is an Abbe number for a d-line.
the objective lens being a single-lens objective lens both surfaces of which are aspherical,
by adjusting a tilt of a recording medium with respect to an optical pick-up device,
the optical pick-up device comprising a light source and the objective lens, for recording or reproducing information on or from the recording medium by irradiating light emitted from the light source onto the recording medium using the objective lens.
According to the foregoing structure, the comatic aberration caused by the manufacturing error can be corrected by adjusting the tilt of the recording medium with respect to the optical pick-up device. Consequently, the aberration can be lessened, and the aberration can be easily corrected.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.